As is well known in the art, lithium ion batteries are recognized as a technology capable of satisfying the requirements all portable devices, including mobile phones, digital cameras, notebook computers and the like, unlike existing batteries, including lead secondary batteries, nickel-cadmium secondary batteries and the like.
In such lithium ion batteries, cathode active materials have the highest added value and constitute the key technology of the lithium ion batteries.
Layered spinel structures such as lithium cobalt oxide (LiCoO2) typical of currently commercially available cathode materials show a high theoretical capacity of 270 mAh/g or higher when 1 mole of lithium reacts. However, in practice, only 0.5 moles of lithium are reacted due to the shortcoming of structural bonds, and thus the theoretical capacity is limited to about 140 mAh/g.
In addition, materials having an olivine structure, such as lithium iron phosphate (LiFePO4), are limited to a theoretical capacity of about 170 mAh/g because of their the high molecular weight of polyphosphate anions. Also it has very low conductivity so that it needs further process for enhance its electrochemical conductivity.
Thus, for application to various devices requiring high performance, the development and use of high-capacity cathode materials is urgently required.
Recently developed lithium transition metal silicon oxides (Li2MSiO4, M=Fe, Ni, Co, Mn etc.) as cathode materials for lithium secondary batteries have 2 moles of lithium and show a high theoretical capacity of 300 mAh/g or higher. Thus, these oxides can provide high capacities compared to other conventional oxides and are receiving a great deal of attention.
Meanwhile, cathode materials for secondary batteries are produced in large amounts by solid-phase reaction methods. In addition, when cathode materials are synthesized by liquid-phase reaction methods, including hydrothermal synthesis and sol-gel methods, it is easy to control particle size and they have excellent electrochemical properties. However, it is very difficult to commercially apply these liquid-phase reaction methods, because starting material costs and process costs are high.
In general solid-phase reaction methods, a solid lithium compound (Li2CO3, LiOH or Li2SiO3), a metal compound (FeC2O4, MnC2O4, MnCO3, MnO2, Mn3O4, Mn(OH)2, NiO or Ni(OH)2) and silica (SiO2) as starting materials are combined and wet- or dry-mixed with each other, and the mixture is calcined at a temperature of 1000° C. or higher, thereby preparing a cathode material.
Although high-temperature heat-treatment is inevitable due to the starting material silica that is very stable even at high temperature, significant amounts of secondary phases (Li2SiO3, MnSiO4, FeSiO4, NiO, MnO2, MnO3 etc.) are present in addition to the lithium metal silicon oxide.
Such secondary phases either do not react with lithium ions or plug lithium-ion migration channels, thus deteriorating the performance of lithium secondary batteries.
Conventional solid-phase methods are generally divided into two methods. One method comprises synthesizing a lithium silicon oxide (Li2SiO3) which is used as a starting material. In this method, to prepare the lithium silicon oxide, heat-treatment is carried out at 400-600° C. for about 30 hours in the presence of carbon dioxide (CO2) and reducing gas (H2) to synthesize lithium silicon oxide powder which is used as a precursor. The synthesized precursor is heat-treated together with manganese oxalate (MnC2O3) in an argon atmosphere.
Other methods include a method comprising a lithium transition metal oxide (LixMOx) at 550-750° C., mixing the lithium transition metal oxide with silicon oxide and subjecting the mixture to solid-phase synthesis. This method is less applicable in synthesis of cathode materials for lithium secondary batteries.
Patent documents which are searched in connection with the background of the invention mostly use crystalline silica as a starting material, resulting in the formation of secondary phases which do not react with lithium ions. Alternatively, these patent documents use expensive lithium silicon oxide (Li2SiO3), and thus are not economical.
Particularly, because methods based on solid-phase methods comprise preparing lithium silicon oxide (Li2SiO3) which is to be used as a starting material, these methods have a shortcoming in that a mixed gas of inert gas and reducing gas is additionally required, making the preparation of the starting material complex and uneconomical.
Other methods include a method in which solid-phase synthesis is performed using oxalate as a starting material. This method can adversely affect the environment when heat treatment is carried out using carbon dioxide (CO2) during the preparation of lithium transition metal silicon oxide.
Accordingly, in these conventional methods, additional reaction materials should be used. Due to the use of these additional reaction materials, the process becomes inefficient and can be risky, and the production cost is increased, and also problems associated with the preparation and treatment of these additional materials occur. Particularly, the above-described solid phase methods are difficult to apply to devices requiring high performance, because it is difficult to control the particle size of synthesized powders.